1. Field of the Invention
The present invention relates to a colored photocurable composition for a solid state image pick-up device, which composition is suitable for producing a color filter used in solid state image pick-up devices (CCD, CMOS, etc.), a color filter and a method for production thereof, and a solid state image pick-up device.
2. Description of the Related Art
Pigments have been conventionally used in a wide variety of applications, as they exhibit vivid color tones and high tinctorial strength. Among these pigments, those presenting practical importance are in general fine particles, and thus vivid color tones and high tinctorial strength are obtained by preventing the aggregation of the pigment particles and finely pulverizing the particles. However, further fine pulverization of the pigment results in an increase in the surface area, whereby particle aggregation is promoted, and thus dispersion liquid of such a pigment often exhibits high viscosity. For this reason, when the pigment dispersion liquid is prepared in an industrial scale, there may occur problems. For example, it is difficult to take out the pigment dispersion liquid from a dispersing machine, transportation through pipelines is not possible, or the pigment dispersion liquid gelates during storage to become unusable.
Thus, it has been known hitherto that various dispersants are used to obtain pigment dispersion liquids or colored photosensitive compositions having excellent fluidity, dispersibility and the like. These dispersants are roughly classified into polymeric dispersants and low molecular weight compound dispersants.
Examples of the polymeric dispersants include polyacrylic acid salts, sodium maleate olefin copolymers, terminal carboxyl group-containing polyesters (see, for example, Japanese Patent Application Publication (JP-B) No. 54-34009), polyesters having an acid group and/or a basic group, prepared from tetrakis(2-hydroxyalkyl)ethylenediamine as a starting material (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2-245231), macromonomers (oligomers having an ethylenic unsaturated group at the terminal), copolymers of four monomers including a monomer having a hydroxyl group, a carboxyl group-containing monomer and monomers other than the monomers (see, for example, JP-A No. 8-259876), and the like.
As for the low molecular weight compound dispersants, sorbitan fatty acid esters, polyoxyethylene alkylamines, alkyldiamines, alkanolamine derivatives and the like are known (see, for example, U.S. Pat. No. 3,536,510), and there are also exemplary dispersants having pigment nuclei introduced thereinto (see, for example, JP-B No. 5-72943, and JP-A No. 8-48890).
Meanwhile, a colored photosensitive composition containing a pigment is useful as a material for color filters used in solid state image pick-up devices, liquid crystal displays and the like, and in the case of producing a color filter using the colored photosensitive composition, a pigment dispersion process which is excellent in terms of quality, production stability or the like, is being widely employed. However, if fine pulverization of the pigment is insufficiently carried out, light is scattered and absorbed by the pigment, and thereby the light transmittance is decreased. Therefore, the contrast is deteriorated, and further, the curing sensitivity at the time of patterning by exposure is lowered (see, for example, a color filter for 10.4-inch TFT-LCD of 512-color display; Ueki, Ozeki, Fukunaga and Yamanaka, the 7th Color Optics Conference, 1990). Furthermore, in the application for a solid state image pick-up device, a decrease in the photosensitivity of the entire sensor caused by the decrease in the light transmittance, and a difference in the particle size of the pigment caused by insufficient fine pulverization, bring about non-uniform decreases in the light transmittance in the respective pixels. Thus, there occurs deterioration in the image quality due to the differences in the sensitivity for the respective pixels. In order to avoid this, it is necessary in a colored photosensitive composition containing a pigment in a dispersed state that the pigment be dispersed in a highly finely pulverized state.
With regard to a color filter for solid state image pick-up device, since reduction of film thickness of colored patterns is in progress for image quality improvement due to high light-concentrating property and high color separation property, the pigment concentration in the composition tends to increase along with this film thickness reduction.
Moreover, as for a pigment-based color filter, color unevenness is likely to occur because the pigment consists of relatively coarse particles. Therefore, as the fine pulverization of pigment is achieved to reduce this color unevenness, the content of pigment dispersant in the curable composition tends to increase. Accordingly, there is a problem concerning the difficulties in obtaining curability.
In order to cope with the problems such as color unevenness in a colored pattern formed, there has been suggested a technology of using an organic solvent-soluble dye as the colorant, in place of pigment (see, for example, JP-A No. 2003-029018). As for such dye-based color filters, an increase in the concentration of dye is accompanied by significant problems such as a polymerization inhibitory effect originating from the dyes, or decrease in stability over time such as dye precipitation.
As described above, in a curable composition for the use in the production of a color filter for a solid state image pick-up device, since the contents of the photopolymerization initiator and the photopolymerizable monomer, which are components needed for curing the curable composition, are limited, and the concentration of the colorant is increased, there have been problems such as that the sensitivity is low, satisfactory curing cannot be obtained, and the adhesiveness to a substrate is insufficient.
In regard to these problems, a technology of improving the sensitivity by imparting polymerizability to a resin which has been introduced mainly to impart film forming property, developability or the like, and other various technologies have been investigated hitherto (see, for example, JP-A No. 2000-321763, JP-A No. 2003-029018, Recent Technological Trend in Color Filters (pp. 85-87, published by Johokiko Co., Ltd.), and Advanced Process Technologies and Chemicals for Color Filters (pp. 129-150, published by CMC Publishing, Ltd.)).
Furthermore, the color filter for the use in solid state image pick-up devices has undergone a pixel size reduction from approximately 5 μm in the late 1990's to 2.2 μm or less in the recent years, in accordance with the demands on an increase in the resolution, an increase in the number of pixels and further micronization of solid state image pick-up devices such as CCD.
As such, as the pixel size is being reduced, there have been raised technical problems concerning the shape of the pixels formed by photolithographic methods. That is, during the production of color filters according to a photoresist method, a photoresist coating film, which is formed by applying on a substrate and colored by a dispersed pigment, is exposed to form a pixel pattern thereon through a photomask, and is developed with alkali to form pixels. However, as the size of the pixels is reduced, the area of contact with the alkali developer solution in the unexposed areas is also decreased. Thus, there are generated residues due to poor solubility, and deterioration in the rectangularity of the pattern shape frequently occurs. The generation of residues leads to a decrease in the sensitivity of adjacent pixels, and more frequent occurrence of tapering results in the mixing of colors in adjacent pixels or in the generation of gaps between pixels. Thus, reproduced images in the solid state image pick-up devices face problems that are prone to impair the image quality, such as that there is an increased noise problem, and the roughness becomes prominent.
As described above, there is a strong demand for an increase in the color density and film thickness reduction in color filters, for the purpose of an improvement in image quality by means of enhanced light-concentrating properties and enhanced color separability of the color filters. When coloring materials are added in large quantities in order to obtain a high color density, the alkali developability is further deteriorated.
In addition, as the photopolymerizable compound in photocurable compositions, acryl esters of dipentaerythritol such as dipentaerythritol hexaacrylate and dipentaerythritol pentaacrylate have been conventionally used. However, these compounds generally have low solubility in aqueous alkali developer solutions, and particularly in the case of forming very fine pixels that are 2.2 μm or less long on one side, the generation of residues due to poor solubility in the unexposed areas has been significant.
In an attempt to improve such alkali developability as described above, there is reported a technology of making use of carboxyl group-containing polyfunctional photocurable compounds or the like (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 10-332929, 2004-287230 and 2005-148717). Meanwhile, a compound produced by introducing an acid group to dipentaerythritol or pentaerythritol is known as a monomer to be incorporated into solder resist (see, for example, JP-A Nos. 8-123027 and 8-123028).
Furthermore, for the purpose of improving the solubility in alkalis, (meth)acrylate compounds having an alkylene oxide structure, such as ethylene oxide or propylene oxide, have been disclosed (see, for example, JP-A No. 10-62986). There are also reports on a photopolymerizable composition containing polyfunctional acrylate such as polypentaerythritol polyacrylate (see, for example, JP-A No. 1-126345), a composition for photopolymerizable solder resist using a polymer of a tetrafunctional acrylate monomer, in which acryloyl groups are linked in an open-ring structure such as ethylene oxide or propylene oxide (see, for example, JP-A Nos. 2-38471 and 64-25147), and the like.
As described above, various technologies have been investigated, but under the actual circumstances, satisfactory sensitivity still cannot be attained. Thus, the problem of film thinning in the exposed areas, or the problem of a decrease in the color density caused by finely pulverized pigment in the exposed areas diffusing into the developer solution, has not been addressed. Since the exposure sensitivity is insufficient, there is also a problem that curing occurs insufficiently in the deeper parts, such as in the vicinity of the interface with a substrate, thereby the adhesiveness to the substrate being poor, and the shape of the pattern becoming inversely tapered. Moreover, for the applications of color filters having a plurality of color patterns, there are also unsolved problems such as that when a pattern of a first color is formed, and then a coating solution for forming a pattern of a second color is applied, the pigment of the first color is diffused into the coating solution of the second color, and the color density is decreased.
The aforementioned carboxyl group-containing polyfunctional photocurable compounds are all constituted to be used in liquid crystal displays. Thus, if a very fine pixel pattern, such as one having a 2.2 μm or less length on one side, is to be satisfactorily reproduced in the application in image sensors, the compounds provide insufficient sensitivity, and have a tendency in general that there occur multiple missing sites in the pattern. Since irradiation with a light of higher energy is required to eliminate these missing sites, the exposure time will be lengthened, and there will be a significant decrease in the yield during the production. On the other hand, the aforementioned solder resist has a low concentration of pigment compared to the resist for color filter, and differs in the blending composition and required performance; therefore, the technology of solder resist cannot be directly applied to the resist for color filter for a solid state image pick-up device.
Also, when the aforementioned (meth)acrylate compounds having an alkylene oxide structure are used to form a very fine pixel pattern (for example, one having a short side of 2.2 μm or less in length) for the use in image sensors and the like, the compounds provide insufficient sensitivity, and have a tendency in general that there occur multiple missing sites in the pattern. Thus, a photopolymerizable composition containing polyfunctional acrylate such as polypentaerythritol polyacrylate, or a composition for photopolymerizable solder resist is not necessarily suitable for the use in solid state image pick-up devices.
As discussed in the above, it is difficult to form, for example, a fine rectangular pixel pattern that is 2.2 μm or less long on one side with a low exposure amount, and to suppress the generation of residues between the pixels, only with the technologies developed so far.